Process for combustion of a liquid hydrocarbon

ABSTRACT

The invention relates to a process for combustion of a liquid Fischer-Tropsch derived hydrocarbon fuel wherein the following steps are performed: (a) obtaining a mixture of liquid hydrocarbon droplets in an oxygen containing gaseous phase; (b) evaporating the liquid hydrocarbon droplets in a cool flame at a temperature of between 300 and 480° C. to obtaining a gaseous mixture comprising oxygen and hydrocarbons; and, (c) total combustion of the gaseous mixture obtained in step (b).

The invention is directed to a process for combustion of a liquidhydrocarbon fuel wherein the following steps are performed:

-   (a) obtaining a mixture of liquid hydrocarbon droplets in an oxygen    containing gaseous phase,-   (b) evaporating the liquid hydrocarbon droplets, preferably in a    cool flame at a temperature of between 300 and 480° C., to obtaining    a gaseous mixture comprising oxygen and hydrocarbons, and-   (c) total combustion of the gaseous mixture obtained in step (b).

Such a process is described in detail in High Modulation Burner forLiquid Fuels Based on Porous Media Combustion and Cool FlameVaporization; D. Trimis, K. Wawrzinek, O. Harzfeld, K. Lucka, A.Rutsche, F. Haase, K. Krüger, C. Kuchen, Sixth International Conferenceon Technologies and Combustion for a Clean Environment (Clean Air VI),Vol. 2, Paper 23.1, Porto, Portugal, 9-12 Jul. 2001. This articledescribes a so-called porous burner, which comprises means to mix airand a liquid fuel, a space for evaporating the liquid fuel in a-coolflame, and a space filled with a porous material in which the combustionof the air/evaporated fuel mixture takes place. As a possible liquidfuel Industrial Gas Oil is mentioned in this article. Means tode-sulphurise the exhaust gas are also present, e.g. in a water-bath;Typical for these types of burners is that they are very suited for lowpower applications in the range of 2 to 30 kW. This makes them verysuited for domestic applications such as domestic heating or boilerapplications. A further advantage is these types of burners allow a highpower modulation of more than 1:10. This allows a reduction ofstart/stop events with its associated temporally higher emissions ofhydrocarbons and carbon monoxide.

A disadvantage of the use of Industrial Gas Oil is that the fuel doesnot easily evaporate in the evaporator space of the burner. Non-completeevaporation of the liquid fuel will result in more emissions in the fluegas leaving the burner. Additionally non-complete evaporation may resultin deposits in the combustion zone and downstream heat exchangersurfaces. This may result in a decrease of efficiency in the heatexchanger, in incomplete combustion or uncontrolled flame ignition.

The objective of the present invention is therefore to provide aprocess, which does not have such a disadvantage. This object isachieved with the following process.

Process for combustion of a liquid Fischer-Tropsch derived hydrocarbonfuel wherein the following steps are performed:

-   (a) obtaining a mixture of liquid hydrocarbon droplets in an oxygen    containing gaseous phase,-   (b) evaporating the liquid hydrocarbon droplets to obtaining a    gaseous mixture comprising oxygen and hydrocarbons, and-   (c) total combustion of the gaseous mixture obtained in step (b).

Applicants found that by using a Fischer-Tropsch derived fuel a betterevaporation takes place in the cool flame. This has resulted in a bettercombustion, improved flame ignition and less fouling of any downstreamheat exchanger surfaces. Furthermore because Fischer-Tropsch derivedfuels contain almost no sulphur no special measures have to be providedto clean the flue gas of said combustion or to apply specialnon-corrosive materials.

In step (a) a mixture of liquid Fischer-Tropsch derived fuel droplets ina gaseous continuous phase is prepared. The gaseous phase will containoxygen or any other oxidant. The gaseous phase is preferably air. Thepreparation of said mixture may be performed by different techniques.For example a mixture is obtained by passing a mixture of air and liquidfuel through small openings at a certain pressure difference resultingin the formation of the small liquid droplets in the gaseous phase. Asecond technique is by atomization of the liquid fuel through ultrasonicvibrations as for example described in U.S. Pat. No. 4,264,837. Apreferred method is wherein the liquid fuel is first atomised by meansof a spray nozzle and subsequently mixed with air as for exampledescribed in the above-cited article.

The size of the droplets will be determined by the method chosen. Incase of a nozzle-the dimensions of the nozzle, the fuel feed rate, fueloil pressure, fuel viscosity (and therefore temperature of the fuel) andsurface tension will influence the droplet size. Smaller droplets andthus a better evaporation of the liquid fuel will be achieved at higherfuel feed rates and/or higher oil pressures for a given feed nozzle.Preferably the droplet size is as small as possible. However the highpressures needed to obtain such small droplets may be not economicallyor technically feasible. Applicants have found that when usingFischer-Tropsch derived fuel larger droplets may be allowed withoutnegatively affecting the combustion. This is very advantageous becausenow a lower oil pressure may be applied which makes the combustionprocess technically more simple and more energy efficient.

The oxygen containing gas will normally be air. However other sources ofoxygen containing gases such as purified oxygen could also be used. Forthe remainder of this description reference shall be made to air,thereby not excluding the alternative sources. The excess air ratio inthe present process is preferably between 1.1 and 3 (excess air ratio isdefined as the ratio between the actual air supply and the needed airfor stochio-metric combustion of the fuel (lambda=l). The liquid fuel ispreferably introduced into the air as a fine spray of droplets.

Step (b) is preferably performed by means of a so-called cool flame.Cool flames, sometimes also referred to as cold flames, start at atemperature of 300° C. and stabilize, virtually independent of the airratio, at a temperature of 480° C. at 1 bar conditions. A cool flamewill be formed when at a certain minimum temperature (300° C.). If thetemperature is kept below 480° C. no auto ignition will occur becausethe needed activation energy is too high under these conditions. Thistemperature is suitably maintained by means of indirect heat exchangeagainst either hot exhaust gasses or against the combustion zone. In thecool flame the liquid droplets will evaporate thereby forming a gaseousmixture that is used in step (c). Steps (b) and (c) in the methodaccording to the present invention are physically separated. Preferablymeasures are taken to avoid hot combustion gases from step (c) to enterthe area wherein the cool flame is present. Examples of such measuresare flame traps through for example flow acceleration or metal gridspositioned at the physical interface between step (a) and step (b).Examples of cool flames are described in the above referred to articleand in EP-A-947769.

Alternatively steps (a) and (b) may be performed by first evaporation ofthe fuel and subsequently mixing the gaseous fuel with the oxygencontaining mixture, or by evaporation in an inert medium before mixingwith the oxygen containing gas.

The combustion in step (c) may be performed in different manners. Forexample aerodynamic stabilization of the flame may be applied. Morepreferably the flame is positioned by means of a porous surface, whereinthe mixture is provided to one end of said surface and a flame ispresent just down stream of said surface. An example of such a surfaceburner is described in EP-A-947769.

Another preferred embodiment for step (c) is wherein the combustiontakes place in a porous material as for example described in the abovereferred to article. The porous material may be as described in theabove-described article or as in U.S. Pat. No. 5522723. It has beenfound important that combustion process may take place inside the porousstructure. Too small pores will quench the flame and too large poreswill cause flame propagation. Preferably the porous material iscomprised of a first zone wherein flame propagation is suppressed, theso-called pre-heating zone and a second zone wherein flame propagationis possible, the actual combustion zone. The porous material may be madefrom for example alumina, zirconium oxide or silicium carbide.

In step (c) preferably a flame detector is used. Examples of suitabledetectors are the UV sensors and IR sensors. A more preferred detectoris the so-called ionisation sensor. An ionisation sensor is suitable tomonitor burners with intermittent operation as well as continuousoperation. The principle of operation of the ionisation flame monitor isbased on the rectifying effect of a flame. If a flame is present, acurrent flows between the burner and the ionisation electrode. Thisionisation current is evaluated by the flame monitor to determine if aflame is present. In some prior art applications ionisation sensorscould not be used in combination with a liquid fuel because deposits inthe sensor led to false currents in the sensor. Because use of theFischer-Tropsch derived fuel, especially a fuel composition notcontaining a metal based combustion improver results in less depositsionisation sensors can be applied. Examples of metal based combustionimprovers are ferrocene based additives andmethylcyclopentadienylmanganese-tricarbonyl (MMT). This is an advantagebecause these sensors are more readily available than the IR or UVsensors.

The Fischer-Tropsch derived fuel will comprise a Fischer-Tropsch productwhich may be any fraction of the middle distillate fuel range, which canbe isolated from the (hydrocracked) Fischer-Tropsch synthesis product.Typical fractions will boil in the naphtha, kerosene or gas oil range.Preferably a Fischer-Tropsch product boiling in the kerosene or gas oilrange is used because these products are easier to handle in for exampledomestic environments. Such products will suitably comprise a fractionlarger than 90 wt % which boils between 160 and 400° C., preferably toabout 370° C.

Examples of Fischer-Tropsch derived kerosene and gas oils are describedin EP-A-583836, WO-A-9714768, WO-A-9714769, WO-A-011116, WO-A-011117,WO-A-0183406, WO-A-0183648, WO-A-0183647, WO-A-0183641, WO-A-0020535,WO-A-0020534, EP-A-1101813, U.S. Pat. No. 5,766,274, U.S. Pat. No.5,378,348, U.S. Pat. No. 5,888,376 and U.S. Pat. No. 6,204,426.

The Fischer-Tropsch product will suitably contain more than 80 wt % andmore suitably more than 95 wt % iso and normal paraffins and less than 1wt % aromatics, the balance being naphthenics compounds. The content ofsulphur and nitrogen will be very low and normally below the detectionlimits for such compounds. This low content of these elements is due tothe specific process wherein the Fischer-Tropsch reaction is performed.The content of sulphur will therefore be below 5 ppm and the content ofnitrogen will be below 1 ppm. As a result of the low contents ofaromatics and naphthenics compounds the density of the Fischer-Tropschproduct will be lower than the conventional mineral derived fuels. Thedensity will be between 0.65 and 0.8 g/cm³.

The fuel used in the process-of the present invention may also comprisefuel fractions other than the Fischer-Tropsch product. Examples of suchfractions may be the kerosene or gas oil fractions as obtained intraditional refinery processes, which upgrade crude petroleum feedstockto useful products. Preferred non-Fischer-Tropsch fuel components arethe ultra low sulphur (e.g. less than 50 ppm sulphur) kerosene or dieselfractions, which are currently on the market. Optionally non-mineral oilbased fuels, such as bio-fuels, may also be present in the fuelcomposition. The content of the Fischer-Tropsch product in the fuel willbe preferably be above 40 wt %, more preferably above 60 wt % and mostpreferably above 80 wt %. It should be understood that the content ofsuch, currently less available, Fischer-Tropsch product will beoptimised, wherein pricing of the total fuel will be balanced with theadvantages of the present invention. For some applications fuels fullybased on a Fischer-Tropsch product plus optionally some additives may beadvantageously used.

The fuel may also comprise one or more of the following additives.Detergents, for example OMA 350 as obtained from Octel OY; stabilizers,for example Keropon ES 3500 as obtained from BASF Aktiengesellschaft,FOA 528A as obtained from OCTEL OY; metal-deactivators, for exampleIRGAMET 30 (as obtained from Specialty Chemicals Inc; (ashless)dispersants, for example as included in the FOA 528 A package asobtained from Octel OY; anti-oxidants: IRGANOX L06, or IRGANOX L57 asobtained from Specialty Chemicals Inc ; cold flow improvers, for exampleKeroflux 3283 as obtained from BASF Aktiengesellschaft, R433 or R474 asobtained from Infineum UK Ltd; anti-corrosion: Additin RC 4801 asobtained from Rhein Chemie GmbH, Kerocorr 3232 as obtained from BASF,SARKOSYL 0 as obtained from Ciba; re-odorants, frQ example Compensol asobtained from Haarmann & Reimer; biociodes, for example. GROTA MAR 71 asobtained from Schuelke & Mayr; lubricity enhancers, for example OLI 9000as obtained from Octel; dehazers, for example T-9318 from Petrolite;antistatic agents, for example Stadis 450 from Octel; and foam reducers,for example TEGO 2079 from Goldschmidt. It has been found that theFischer-Tropsch derived fuel does not necessarily have to contain acombustion improver such as for example ferrocene or MMT.

The Fischer-Tropsch product is colourless and odourless. For safetyreasons an odour marker, as for example applied in natural gas fordomestic consumption, may be present in the Fischer-Tropsch derivedfuel. Also a colour marker may be present to distinguish the fuel fromother non-Fischer-Tropsch derived fuels.

The total content of the additives may be suitably between 0 and 1 wt %and preferably below 0.5 wt %.

The combustion process using the Fischer-Tropsch fuels is preferablyapplied for domestic heating, wherein the heat of combustion is used toheat water by indirect heat exchange in so-called boilers. The processis especially suited for domestic applications because of its powermodulation range of between 2 and 30 kW. The heated water may be used towarm up the house or consumed in for example showers and the like.

The combustion process using the Fischer-Tropsch fuels mayadvantageously be further used for direct heating of large spaces. Suchapplications are characterized in that the flue gasses are directlysupplied to said space to heat up said space. Spaces such a tents andhalls are often heated up with such an apparatus. Normally gaseous fuelsfor example natural gas, LPG and the like, are used for this applicationbecause the associated flue gasses can be safely supplied to said space.A disadvantage of the use of gaseous fuels is however that handling ofthe pressurized gas containers and combustion equipment requiresprofessional skills in order to operate such an apparatus safely. Byusing a Fischer-Tropsch derived liquid fuel a comparable flue gas isobtained in the combustion process as when a gaseous fuel is used. Thusa method is provided wherein a liquid fuel can be applied for directheating of spaces. The application of the liquid Fischer-Tropsch derivedfuel makes the use of the apparatus for direct heating much more simpleand safe.

The direct heating of spaces is preferably performed by means of aso-called radiation heater. In such an apparatus step (c) is preferablyperformed at or in the surface of a perforated plate. This plate ispreferably a ceramic plate. In this plate combustion will take-place inthe short channels across the plate. The combustion heat will result ina glowing plate generating radiation energy that will heat up thesurrounding air. Typically the fuel for such radiation heaters is agaseous fuel because the flue gasses will also be emitted in thesurrounding air. Applicants have now found that also a Fischer-Tropschfuel may be advantageously applied without the disadvantages of a liquidfuel. Examples of radiation heaters which normally operate on a gaseousfuel but which now can be operated on a liquid fuel are described inU.S. Pat. No. 5,139,415, EP-A-0949452 or EP-A-0037046.

The combustion process using the Fischer-Tropsch fuels is preferablyapplied for domestic heating, wherein the heat of combustion is used toheat water by indirect heat exchange in so-called boilers. The processis especially suited for domestic applications because of its powermodulation range of between 2 and 30 kW. The heated water may be used towarm up the house or consumed in for example showers and the like.

The combustion process using the Fischer-Tropsch fuels mayadvantageously be further used for direct heating of large spaces. Suchapplications are characterized in that the flue gasses are directlysupplied to said space to heat up said space. For this application step(c) is preferably performed using a porous surface. The radiant heatdeveloped at the surface of such heaters will heat the environment inwhich it is placed.

The process may also be advantageously applied in a process to generatesteam. Especially when step (c) is performed in a porous material asdescribed in for example U.S. Pat. No. 5,522,723. The heat of combustiongenerated by such process can be used to generate steam, which may beused for various purposes, such as heating. A preferred application isdescribed in U.S. Pat. No. 2002194848 and WO-A-03036072, wherein saidgenerated steam is first super heated and subsequently fed to a pistonengine or an expansion engine. This application is sometimes alsoreferred to as the SteamCell of Enginion AG (SteamCell is a trademark).The engine may provide mechanical power, for example to power anautomobile, or electricity. The claimed advantage of this type of engineis low No_(x) emissions as compared to the state of the art combustionengines. By using a Fischer-Tropsch derived fuel NO_(x) emissions may befurther reduced in such an application. An additional advantage is thatthe Fischer-Tropsch derived fuel is practically sulphur free. This canfurther simplify the design of the burner and reduce the complexity ofsuch an engine.

1. A process for combustion of a liquid Fischer-Tropsch derivedhydrocarbon fuel wherein the following steps are performed: (a)obtaining a mixture of liquid hydrocarbon droplets in an oxygencontaining gaseous phase, (b) evaporating the liquid hydrocarbondroplets to obtaining a gaseous mixture comprising oxygen andhydrocarbons, and, (c) combusting completely of the gaseous mixtureobtained in step (b) to produce a heat of combustion.
 2. The process ofclaim 1, wherein step (a) is performed by atomization of the liquidFischer-Tropsch derived fuel by means of a spray nozzle and subsequentlymixing the atomized fuel with air.
 3. The process of claim 1, whereinstep (b) is performed in a cool flame at a temperature of between 300°C. and 480° C.
 4. The process of claim 1, wherein step (c) is performedin a porous material.
 5. The process of claim 4, further comprising: (d)producing steam from the heat of combustion from step (c); (e) superheating the steam; and, (f) powering piston or expansion engine with thesuperheated steam.
 6. The process of claim 1, wherein step (c) isperformed at a porous surface to produce radiant heat.
 7. The process ofclaim 6, further comprising heating spaces with the radiant heat at theporous surface.
 8. The process of claims 1, wherein step (c) furthercomprises aerodynamically stabilizing the flame.
 9. The process ofclaims 1, wherein the fuel comprises a Fischer-Tropsch productcomprising more than 80 wt % iso and normal paraffins.
 10. The processof claim 9, wherein the fuel comprises more than 80 wt % ofFischer-Tropsch product.
 11. The process of claims 1, wherein the fueldoes not contain a metal based combustion improver and wherein in step(c) a flame detector is present of the ionization sensor type.